As a transmission system for a multi air conditioner for building use, a device is conventionally known in which a plurality of air conditioning units each are so composed that one outdoor unit is connected to a plurality of indoor units through an internal/external transmission line while each outdoor unit of the plurality of air conditioning units is connected to a centralized control unit through an integrated transmission line (see Patent Document 1, for example).
In the transmission system in Patent Document 1, the centralized control unit and the outdoor units are bus-connected to each other through the integrated transmission line. When the integrated transmission line and communication lines for the indoor units use the same protocol, the number of communication interfaces can be reduced to one. The integrated transmission line and the communication lines for the indoor units are connected to each other via a relay or the like inside each outdoor units, and accordingly, the indoor units can be regarded to be bus-connected to the communication lines branching from the integrated transmission line when viewed from the integrated transmission line.    Patent Document 1: Japanese Patent No. 3206476